It is also listed as an uncorrected bug page 571.
This is not a bug.
Actually, the code here is trying to validate the drive letter asked by the user. But there are two ways to ask for a drive:
1- entering "A-" (or B- or C- or D- ) will set the default drive to the chosen letter. Any file load attempt will be made from this default drive
2- loading a file specifying the drive where it's stored, therefore bypassing the default drive, for instance: LOAD"B-ZAKMCKRAK" will load the file ZAKMCKRAK on the drive B, whatever the default drive is (and providing Chema has ported the game

So the two JSR D7BD are there for these two ways of asking for a drive: one will just be used to load a file, the other will set the default drive. They are two different entries, each one requiring (more or less, but that's another story) the JSR D7BD.